Openable structures of the mentioned kind are typically in the form of for example top-hung windows installed in inclined surfaces, e.g. a roof of a building, the first member being a side sash member, and the second member being a side frame member of said window, or vice versa. A respective lifting device with a spring-biased slide shoe connects a respective side frame member and a respective sash side member on each side of the window. It is an aim in such windows to ensure that the force needing to be exerted by a user on the window sash when opening or closing of the window does not vary too much during the travelled path of the window sash, no matter of the roof inclination of the roof in which the window is installed and no matter variance of the weight of the window sash. The window sash weight may vary due to for example installation of optional accessories such as miscellaneous screening devices, e.g. blackout curtains, some of which can be installed on mounted windows. In an attempt to alleviate this, applicant's EP 0 733 146 A suggests to provide an adjustable auxiliary spring in the lifting device to increase the spring bias within certain opening angles in windows mounted in roofs with certain roof inclinations.
The biased slide shoe of such lifting devices is mounted on and moves in parallel with or along either the side frame member or the side sash member. The slide shoe typically slides in a guidance mounted on the respective sash or frame member. The slide shoe is typically biased by means of a spring arrangement located in parallel with the respective sash or frame member, and pivotally connected to a lifting arm, the other end of which is pivotally connected with the other of the side sash member or side frame member. During opening and closing of the window the slide shoe slides in the guidance, providing a frictional braking force normal to a guidance bottom face, which in turn allows for the window to be placed in positions between fully opened and fully closed. When the window is opened, the lifting arm moves from a position near parallel with the frame or sash member on which the guidance is mounted to a position with an angle to both of these. In the angled position the frictional force exerted by the slide shoe on the guidance, and hence the braking effect, is inherently somewhat larger. Conversely, because the frictional force from the slide shoe normal to the guidance bottom face decreases during closing of the window, in a near-closed position of the window the braking effect is lower, making the window sensitive to influences such as wind forces in the near closed position. Thus, it is also an aim in such windows that with any roof inclination and weight of optional accessories mounted on the window sash the frictional or braking forces working during operation are such that the window can be firmly positioned in any opening degree, i.e. without the risk of sliding out of position because of for example gravitational or wind-induced forces exerted on the movable part of the window.
EP 1 052 342 B suggests as a solution to the latter problem a lifting device, in which a braking effect of a braking device is adjustable by means of a threaded bolt. The threaded bolt adjusts the force exerted by two lateral brake shoes on respective lateral brake faces of a guidance of the lifting device. This makes it possible to add a constant braking effect exerted in the entire path between fully opened and fully closed positions of the window, this added braking effect not being dependant on the bias on the slide shoe or the position of the window. If this braking device is adjusted to be able to hold the movable window part in a near-closed position, the accumulated braking effect is larger than desired in positions of larger opening angles, making it too hard for a user to bring the window from one position to another within these angles. Further, this braking device is subjected to wear and frequently needs to be replaced or readjusted.
A lifting device according to the introductory part of claim 1 and suggesting another solution to the problem of providing a suitable braking force in all positions of the window is known from applicant's EP 1 873 323 A. In this lifting device, the slide shoe comprises a braking device, which modulates a force resulting from the bias on the slide shoe in variation of a magnitude of said force, i.e. in variation of the path of the slide shoe. This provides for a path-dependent braking effect, which can be arranged to ensure that the braking effect is greater within the opening angles in which it is needed (i.e. near-closed window). The braking device comprises a linkage mechanism having two brake members connected with each other by a link extending at an angle with respect to a guidance portion forming at least one face in contact with a respective brake member. However, a window provided with this lifting device is only suitable with windows mounted in roofs with a certain interval of roof inclination angles and with window sashes of a certain weight.
DE 2 337 459 A discloses a lifting device in which a brake faces of a guidance, on which a slide shoe slides, are provided converging against each other along the path of the slide shoe. This provides a comparable braking effect as is disclosed in EP 1 873 323 A.
Thus, despite of these prior art solutions somewhat alleviating the mentioned problems, a need for further improving the ability of such a lifting device to adjust to different installation situations of an openable structure, such as a roof-mounted window, continues to exist. Further, lifting devices taking up less space, providing less wear and being cheaper to manufacture are desired.